1. Field of the Invention
The present invention relates to a composite electrode including an organic disulfide compound that is suitable for use in an electrochemical element such as a battery, an electrochromic display element, a sensor or a memory. The present invention also relates to a method of producing the composite electrode and a lithium secondary battery utilizing the composite electrode as a cathode (positive electrode).
2. Description of the Prior Art
Ever since the discovery of conductive polyacetylene in 1971, various studies have been made on conductive polymer electrodes because the usage of a conductive polymer as an electrode material has been considered suitable for a variety of electrochemical elements such as a light weight battery with a high energy density, an electrochromic element with a large surface area, and a biochemical sensor using a microelectrode. Since polyacetylene is less practical because of its instability, other .pi.-electron-conjugated conductive polymers have been sought for these uses. As a result, comparatively stable polymers such as polyaniline, polypyrrole, polyacene and polythiophene have been found, and a lithium secondary battery using a cathode made of any of these polymers has been developed. Such a battery is estimated to have an energy density of 40 to 80 Wh/kg.
Recently, U.S. Pat. No. 4,833,048 described an organic disulfide compound as an organic material which can attain a further higher energy density. This compound is represented, in its simplest form, by the formula M.sup.+ --.sup.- S--R--S.sup.- --M.sup.+, wherein R indicates an aliphatic or aromatic organic residue, S indicates a sulfur atom and M.sup.+ indicates a proton or metallic cation. Such compounds bind to each other via S--S bonds by electrolytic oxidation, and form a polymer represented by M.sup.+ --.sup.- S--R--S--S--R--S--S--R--S.sup.- --M.sup.+. The thus formed polymer is regenerated into the original monomer by electrolytic reduction. The aforementioned U.S. Patent discloses a metallic-sulfur secondary battery using a metallic M which supplies and captures cations (M.sup.+) in combination with an organic disulfide compound. This battery can realize a very high energy density of 150 Wh/kg or higher, which is comparable to or above the energy density of an ordinary secondary battery.
The use of such an organic disulfide compound in an electrode, however, has a problem that repeated electrolytic oxidation/reduction (i.e., charge/discharge) results in a gradual reduction of the electrode capacity. Oxidation (charge) of the organic disulfide compound generates a polydisulfide compound which is electrically insulating and has a poor ionic conductivity. The polydisulfide compound has low solubility in an electrolyte. In contrast, an organic disulfide monomer which is generated by reducing (discharging) the polydisulfide compound has high solubility in an electrolyte. Therefore, when the oxidation and reduction is repeated, part of the disulfide monomer is dissolved in the electrolyte, and the dissolved monomer is polymerized at a site different from its original position in the electrode. Thus, the polydisulfide compound precipitated away from an electrically-conductive agent such as carbon is isolated from the electron/ion conductive network in the electrode and makes no contribution to the electrode reaction. Repeated oxidation and reduction increases the amount of isolated polydisulfide compound, resulting in a gradual decrease in the capacity of the battery. In addition, the organic disulfide monomer with high solubility tends to move, and hence, it can dissipate from the cathode into the separator or the electrolyte and further toward the anode (negative electrode). As a result, the battery using the organic disulfide compound for its cathode has disadvantageously low charge/discharge efficiency and a short charge-discharge cycle life.